1. Field of the Invention
This invention relates to apparatus for optically recording or printing an image on a recording medium, and in particular, to an optical recording apparatus including an array of light-emitting elements for recording an image on a belt-shaped recording medium.
2. Description of the Prior Art
An optical recording apparatus for optically recording an image on a recording medium, such as a photosensitive member, is well known in the art. FIG. 1 illustrates one such prior art optical recording apparatus employing a self-scanning type optical recording head 1. Also provided is an endless photosensitive belt 4 as extended between a pair of rollers 2 and 3, at least one of which is driven to rotate in the direction indicated by the arrow to cause the belt 4 to run at constant speed. As the belt 4 travels, its outside surface is uniformly charged by a corona charger 5 which is disposed upstream of the optical recording head 1 with respect to the travelling direction of the belt 4.
As will be understood later, the recording head 1 is provided with a plurality of light-emitting elements arranged in the form of a single array, which are selectively activated by a driver circuit (not shown) connected to the recording head 1 in accordance with an electrical image signal supplied thereto from an external circuitry thereby emitting light to be impinged upon the uniformly charged belt 4 in motion. As a result, the charge on the belt 4 is selectively dissipated when the light thus emitted strikes the belt 4 so that an electrostatic latent image is formed on the belt 4 as it moves past the recording head 1. Then the latent image moves past a developing station 6 where the latent image is converted into a visible image, such as a toner image.
The toner image thus formed at the developing station 6 is transported toward an image transfer station, where a transfer corona unit 11 is disposed, as riding on the moving belt 4. On the other hand, a sheet of transfer paper 10 stored in the form of a stack is also transported toward the image transfer station by means of a feed roller 7, a sheet separation roller 8 and a registration roller 9 in association with the operation of the belt 4. The sheet of transfer paper 10 is then brought into contact with the outer surface of the belt 4 on which the toner image is supported so that the toner image is transferred to the sheet of transfer paper 10 as separated from the belt 4 due to corona ions applied to the back side of the sheet of transfer paper 10 as it moves past the image transfer station. The sheet of transfer paper 10 now bearing thereon the transferred toner image is then moved past an image fixing station where the transferred toner image is permanently fixed to the sheet of transfer paper 10, which is thereafter discharged outside of the apparatus by means of paper discharging roller 13. On the other hand, after transfer of toner image to a sheet of transfer paper 10, the belt 4 receives a uniform irradiation from a charge removing lamp 14 so that the remaining charge on the belt 4 is removed. Thereafter, the belt 4 is cleaned by a cleaning device 15 to remove any unwanted material remaining on the belt 4 thereby preparing the belt 4 to be ready for the next cycle of operation.
In the illustrated example, use is made of a photosensitive member in the form of an endless belt. In such a case, the overall apparatus may be designed to be flat and compact in size and there is a high degree of freedom in arranging various image formation-related elements, such as charging, image exposing, developing and cleaning units. It is to be further noted that disposal is easier for belt-shaped photosensitive members as opposed to drum-shaped photosensitive members.
As described above, the optical recording head 1 of the illustrated example is of the self-scanning type in which a number of light-emitting elements, such as light-emitting diodes, or optical shutters, such as liquid crystal shutter elements, are arranged in the form of a single array. Instead of using such a self-scanning type recording head, use may also be made of a recording head having a single light source, such as a laser. When the laser is used, however, there must be provided such elements as oscillation control unit and a laser beam deflecting unit. Thus, in general, the laser recording head tends to be expensive and complicated in structure as compared with a self-scanning type recording head. The self-scanning type recording heads may be generally classified into two groups: light-emitting type, such as using a light-emitting diode array and a fluorescent tube array, and non-light-emitting type, such as using a liquid crystal display array.
FIG. 2 shows one example of the prior art light emitting type self-scanning optical recording head 1A. As shown, the recording head 1A includes a support plate 16, also serving as a heat sink, of aluminum or the like and a substrate 17 fixedly attached to the bottom surface of the support plate 16. On the bottom surface of the substrate 17 is provided a light source array 18 comprised of a plurality of light-emitting elements, such as light-emitting diodes and fluorescent light tubes. On both sides of the light source array 18 are provided driver I.Cs. 19 as also fixedly attached to the bottom surface of the substrate 17. Also provided are leads 20 and connectors 21 mounted on the top surface of the support plate 16 for connecting the I.C. drivers 19 to external circuitry. Below the substrate 17 is disposed a holding member 23 with a seal member 22 in the form of a closed loop sandwiched therebetween, and the holding member 23 holds an image-forming or focusing device, such as Selfoc Lens Array commercially available from Nihon Itagarasu, Inc. of Japan, in position between the light source array 18 and the photosensitive belt 4. It is to be noted that the focusing device 24 is so provided to be movable up and down in fine mode for focusing operation.
FIG. 3 on the other hand illustrates an example of the prior art non-light-emitting type self-scanning optical recording head 1B employing an LCD panel 27. As shown, the recording head 1B includes a single light source 25, the light from which is collected onto the LCD panel 27 by means of a rod lens 26 and the light passing through the panel 27 is focused onto the belt 4 by means of the focusing device 24. In this example, the LCD panel 27 and the driver I.Cs. are mounted on the same supporting structure.
As described above, in either type, use is made of the focusing device 24 to have an image focused on the belt 4. The focusing device, in particular Selfoc Lens Array used for line scanning has a rather shallow field of depth. On the other hand, the photosensitive belt 4 is often times subjected to undulating motion, whereby the belt 4 partly moves up and down in the direction normal to its transporting direction. Thus, in the case where the Selfoc Lens Array 24 is used to have an image focused onto the belt 4, the image becomes defocused in a periodic manner due to undulating motion of the belt 4 while in operation, so that the quality of printed image tends to become deteriorated.